Selective NOx catalytic reduction system including an ammonia sensor

ABSTRACT

An improved SCR system for controlling NOx levels in internal combustion engine exhaust, comprising a least one ammonia sensor disposed at an intermediate longitudinal location in an SCR catalyst and in communication with a System Control Module (SCM). The ammonia measurement permits calculation of ammonia storage on catalyst sites via a stored SCM algorithm. Locating the ammonia sensor midway in the catalyst allows for optimum control of NOx reduction and permits the portion of the catalyst downstream of the sensor to be treated as a slip catalyst, thus minimizing or eliminating the need for a second slip catalyst and housing, and reducing the size, volume, complexity, and cost of an SCR system. In-brick ammonia sensor permits the system to manage engine exhaust to a desired NOx conversion level and ammonia slip target value, thus minimizing the rate of consumption of ammonia while meeting required limits for NOx emissions.

TECHNICAL FIELD

The present invention relates to systems for controlling the level of nitrogen oxides (NOx) in internal combustion engine exhaust; more particularly, to such systems for catalytically reducing NOx to N₂ by reaction with ammonia; and most particularly, to an improved system having an ammonia sensor disposed within the catalyst for feedback control to manage exhaust levels of NOx and ammonia.

BACKGROUND OF THE INVENTION

Reducing and controlling engine emissions of oxides of nitrogen are important considerations in modern internal combustion engines, both spark-ignited and compression-ignited. NOx emissions are an element of smog production, and emissions limits mandated by state governments and/or the federal government are likely to become even more stringent in the future.

One known approach to reducing NOx emissions is to reduce NOx formation by reducing combustion temperatures, such as by recirculation of exhaust gas into the engine firing chambers to dilute the combustion mixture. Even under the best of control, however, untreated engine exhaust typically contains an unacceptable level of NOx. Thus, another approach is to strip NOx from the exhaust via one or more aftertreatment devices.

Aftertreatment systems are known in the art which can convert NOx to elemental N₂ by selective catalytic reduction (SCR) in the presence of a suitable reductant, for example, ammonia (NH₃) in accordance with the following equations:

NO+NO₂+2NH₃→2N₂+3H₂O   (Eq. 1)

4NO+O₂+4NH₃→4N₂+6H₂O   (Eq. 2)

2NO₂+O₂+4NH₃→3N₂+6H₂O   (Eq. 3)

Typically, ammonia is provided (“dosed”) to the catalyst via decomposition of urea (typically aqueous urea) in accordance with the following equations:

CO(NH₂)₂ (aqueous)→CO(NH₂)₂+H₂O   (Eq. 4)

CO(NH₂)₂→NH₃+HNCO   (Eq. 5)

HNCO+H₂O→NH₃+CO₂   (Eq. 6)

or a net reaction of:

CO(NH₂)₂+H₂O→2NH₃+CO₂   (Eq. 7)

It will be recognized that specific molar match of ammonia to NOx is desired to convert all NOx while slipping no excess ammonia to atmosphere. In practice, this has proved to be very difficult to achieve. For example, in a prior art SCR system first and second catalyst bricks are required, typically disposed in two separate sequential housings. The first brick is designated as the SCR catalyst and the second brick is designated as the “slip” catalyst for oxidizing residual exhaust ammonia, which is known to happen due to one or more of three causes:

First, excess ammonia in the tailpipe exhaust can be due to incomplete reaction of the SCR as shown in Eqs. 1-3.

Second, in the SCR catalytic reaction mechanism, NOx reacts with ammonia stored on the catalyst. Ammonia storage capacity is highly dependent on temperature of the catalyst, with capacity at low temperatures being significantly greater than at higher temperatures. Because of this effect, even when dosing is greatly reduced or even stopped completely during hot exhaust transients, unreacted ammonia can be desorbed from the SCR catalyst and pass into the tailpipe exhaust.

Lastly, dosing at low temperatures can lead to solid or liquid urea deposits in the exhaust system which, upon subsequent heating, can lead to additional ammonia release unaccounted for in the dosing control.

The slip catalyst acts to oxidize excess ammonia, converting it into elemental nitrogen, per the following mechanism:

4NH₃+3O₂→2N₂+6H₂O   (Eq. 8)

Less desirably, excess ammonia may also be further oxidized back into NOx, per the following mechanisms:

4NH₃+5O₂→4NO+6H₂O   (Eq. 9)

2NH₃+2O₂→N₂O+3H₂O   (Eq. 10)

A prior art SCR system may further include, after the SCR or slip catalyst, an NOx sensor which is additionally cross-sensitive to ammonia, and the system is close-loop controlled to maintain slipped ammonia below a predetermined level by regulating the dosing rate of aqueous urea.

Such a prior art SCR system has several shortcomings. First, significant inadequacies in the prior art require a slip catalyst element and volume of slip catalyst to offset lack of optimal dosing control. Second, due to hysteresis the system cannot be responsive to abrupt changes in NOx load or catalyst temperature, which occur frequently in actual engine usage. Third, when the sensor is placed after the slip catalyst, ammonia sensed by the ammonia sensor is by definition lost to atmosphere, as the system has no means for absorbing or oxidizing slipped ammonia after the sensor.

What is needed in the art is an improved method and apparatus for determining and controlling the ammonia level resident in the exhaust leaving the SCR catalyst.

What is further needed in the art is an improved method and apparatus for eliminating the need for a slip catalyst or minimizing slip catalyst volume as well as minimizing the amount of slip ammonia lost to atmosphere.

It is a principal object of the present invention to optimize the consumption of urea in an SCR system to a targeted ammonia slip average and/or slip maximum while meeting required targets for NOx emissions.

It is a still further object of the present invention to reduce the size, volume, complexity, and cost of an SCR system.

SUMMARY OF THE INVENTION

Briefly described, an improved SCR system in accordance with the invention comprises a least one ammonia sensor disposed at an intermediate longitudinal location in an NOx-reducing SCR catalyst and in communication with an Engine Control Module (ECM). Locating the ammonia sensor within the catalyst allows for optimal NOx reduction and permits the downstream NOx catalyst to be treated as an effective slip catalyst, thus minimizing or eliminating the need for a second slip catalyst and housing, and reducing the size, volume, complexity, and cost of the SCR system. Continuous measurement of ammonia concentration in exhaust at an intermediate location in the SCR catalyst permits calculation of ammonia storage amounts on active catalyst sites throughout the catalyst brick, and thus permits the engine exhaust to be managed to a desired NOx conversion level and ammonia slip target value. Further, placing the ammonia sensor closer to the point of urea introduction reduces hysteresis in the SCR system, allowing faster response to NOx load changes.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an elevational view, partially in cutaway, of the exhaust treatment catalysts in a prior art SCR;

FIG. 2 is a schematic drawing of an SCR system improved in accordance with the present invention;

FIG. 3 is a graphical representation of key factors in SCR control; and

FIGS. 4 a, 4 b, and 4 c are elevational views, partially in cutaway, of three alternative embodiments of the improved SCR catalytic converter shown in FIG. 2.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein are not to be construed as limiting the scope of the invention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The benefits and advantages of the present invention may be better appreciated by first considering a prior art SCR system.

Referring to FIG. 1, a catalyst assembly 10 in a prior art SCR system comprises a Selective Catalytic Reduction (SCR) unit 14 and an optional ammonia slip catalyst 16 connected in sequence to treat raw engine exhaust 18 from an internal combustion engine 20, especially a diesel or gas engine operated with excess oxygen in the exhausted gases, and to discharge treated engine exhaust 22 to atmosphere 24.

SCR 14 includes a first selective catalyst 30 disposed in a first housing 32 for selectively reducing NOx to N₂ in the presence of NH₃ and O₂, as described above, in known fashion. An atomizing nozzle 34 prior to SCR 14 receives a reductant solution 38 containing urea and/or ammonia from a source 40 and sprays atomized reductant solution 42 into exhaust 18.

Slip catalyst 16, a second selective catalyst, is disposed in a second housing 46 for receiving intermediate exhaust 48 containing excess ammonia which is oxidized by catalyst 16 to minimize the amount of ammonia slipped to atmosphere 24 in treated exhaust 22. An exhaust gas sensor 50 sensitive to both NOx and NH₃ is disposed between first and second SCR housings 32,46 and monitors levels of NOx and NH₃ in intermediate exhaust 48 and communicates with a System Control Module 54 for feedback control of dosing rate of reductant solution 38 from source 40 based upon the sensed NOx levels in the exhaust. (Description and illustration of the storage and dosing apparatus is omitted for clarity.) An optional ammonia sensor 56 may be provided after slip catalyst 16 to monitor actual slip ammonia levels in treated exhaust 22.

Referring now to FIG. 2, an exemplary improved SCR system 100 in accordance with one aspect of the present invention comprises a dosing apparatus assembly 102, a catalyst assembly 110, and a system controller 154.

Dosing apparatus assembly 102 includes a tank 158 for storing reductant solution 38, a high level sensor 160, a temperature sensor 164, and a solution heater 166 in communication with system controller 154 which may be an Engine Control Module. A supply line 168 leading from a solution filter 170 and containing a pressure sensor 172 is connected to catalyst assembly 110 at a solenoid pump/injector 174 for supplying reductant solution 38 to atomizer 134. Preferably, supply line 168 is heated by an electric heater 176.

Some elements of catalyst assembly 110 are similar to their counterparts in prior art catalyst assembly 10. Assembly 110 comprises a Selective Catalytic Reduction (SCR) unit 114 comprising both a selective NOx reduction first catalyst 130 and a second catalyst 116. Catalysts 130,116 may be disposed in separate sequential housings as in the prior art but preferably are disposed in a common housing 132, all elements being connected in sequence to treat raw engine exhaust 18 from an internal combustion engine 20, especially a diesel engine, and to discharge treated engine exhaust 122 to atmosphere 24.

Preferably, catalysts 116,130 are provided as porous or channeled monoliths known in the art, and used herein, as “bricks”.

Selective catalyst 130 is disposed in housing 132 for selectively reducing NOx to N₂ in the presence of NH₃ and O₂, as described above. Atomizing nozzle 134 sprays atomized reductant solution 42 into exhaust 18.

(Note that an SCR system in accordance with the invention contemplates provision of ammonia from any source. For exemplary purposes, the present discussion refers only to urea as the source, and to liquid urea dosing as the means of introduction, but it should be understood that all other ammoniacal chemical reductants and apparatus for supplying them having the net effect of providing ammonia to the SCR catalysts are fully comprehended by the invention.)

Second catalyst 116, preferably also disposed in housing 132 and downstream of selective NOx catalyst 130, receives treated exhaust which may contain slip ammonia and unreacted NOx.

In a first embodiment (see FIG. 4 a), catalyst 116 a is an extension of selective NOx catalyst 130 a, and slip ammonia is controlled as described below.

In a second and third embodiments (see FIGS. 4 b and 4 c), second catalysts 116 b, 116 c are selective NOx catalysts similar to catalysts 130 b, 130 c. However, optionally, catalysts 116 b, 116 c may also have ammonia-oxidizing capability like a prior art ammonia slip catalyst. In this configuration, excess ammonia is oxidized by catalysts 116 b, 116 c, and NOx passing beyond catalysts 130 b, 130 c is also reduced by catalysts 116 b, 116 c, thereby minimizing the amount of ammonia and NOx slipped to atmosphere 24 in treated exhaust 122.

An important feature of the present invention is an ammonia sensor 180 disposed between first and second catalysts 130,116 and in communication with a System Control Module (SCM) 154 for feedback control of dosing rate of reductant solution from tank 158. As used herein, the term “ammonia sensor” should be taken to mean any device capable of determining the ammonia content in exhaust gases. Note that prior art catalyst assembly 10 becomes a functional embodiment of the present invention when provided with an ammonia sensor 180 in place of NH₃/NOx sensor 50 in accordance with the present invention, even though catalysts 30 and 16 are disposed in separate housings.

Exemplary ammonia sensors are disclosed in the prior art; see, for example, U.S. Pat. No. 7,069,770 B2, and Published US Patent Application Nos. 20060200969 A1 and 20070080074 A1, the relevant disclosures of which are incorporated herein by reference. Such sensors are presented as only exemplary, and any other ammonia sensing device is fully anticipated by the invention.

The present invention affords at least two distinct advantages.

First, sensing ammonia rather than nitrogen oxides in the treated exhaust permits direct feedback flow control of ammonia-generating reductant via an algorithm programmed into SCM 154. The algorithm factors in at least the exhaust temperature at the first catalyst inlet (temperature sensor 182), the exhaust temperature at the second catalyst outlet (temperature sensor 184), the flow rate of reductant, and the instantaneous concentration of exhaust ammonia, as well as various engine operating parameters, to calculate the new flow rate.

Second, placing the ammonia sensor ahead of second catalyst 116 allows the algorithm also to calculate the instantaneous ammonia loading already in both catalyst bricks 116,130, as a part of calculating a new reductant flow rate, and further allows ammonia-oxidizing catalysts 116 b, 116 c to oxidize ammonia sensed by sensor 180. Thus, the algorithm is able to minimize consumption of reductant and formation of atmospheric slip ammonia while maximizing conversion of NOx to N₂ in treated exhaust 122. Note that an optional second ammonia sensor 186 after second catalyst 116 may also be included to monitor tailgas ammonia levels in treated exhaust 122 directly, to confirm proper operation of the control system 100.

Referring to FIG. 3, the sensing and control advantages of the present invention are shown. The actual concentrations of ammonia and NOx in tailgas exhaust 122 are shown as a function of the rate of urea dosing. As ammonia concentration (curve 188) increases, NOx concentration (curve 190) decreases. By severely overdosing the system with urea, the concentration of NOx can be reduced to near zero (SCR efficiency=100%) but with a large amount of slip ammonia being released to atmosphere. Use of an ammonia sensor permits correlation of curves 184,186 at all points such that actual levels of ammonia and NOx in exhaust 122 are known, and the system may be controlled to any desired value of either one. On the other hand, in prior art systems (FIG. 1) herein a tailgas sensor 50 (FIG. 3 curve 192) is sensitive to both NOx and NH₃, the sensor output is the same for two values of the curve, e.g., points 194,196, and therefore cannot distinguish whether to further increase or to decrease urea flow.

A third advantage of the present invention is that both first catalyst 130 and second catalyst 116 may be disposed together in a single housing 132, thus reducing the cost and volume of an SCR system.

Referring to FIGS. 4 a-4 c, various configurations of catalysts and ammonia sensor are possible within the scope of the present invention. In each case, the first and second catalysts 130,116 have NOx-reducing capability, and optionally second catalysts 116 may also have ammonia-oxidizing capability.

FIG. 4 a shows a single monolithic brick catalyst disposed in housing 132 and having a well 198 formed at an intermediate longitudinal location for receiving ammonia sensor 180, which location defines portions of the brick upstream of sensor 180 as SCR catalyst 130 a and portions downstream of sensor 180 as slip catalyst 116 a.

FIG. 4 b shows SCR catalyst 130 b and slip catalyst 116 b abuttingly disposed in housing 132 and having a well 198 formed at the abutting location for receiving ammonia sensor 180.

FIG. 4 c shows SCR catalyst 130 c and slip catalyst 116 c sequentially disposed in housing 132 and having a gap 199 formed therebetween for receiving ammonia sensor 180.

In operation, ammonia, preferably formed from a urea solution, is supplied (dosed) via nozzle 134 and exhaust 18 into SCR catalyst assembly 114 at a controlled dispensing rate. Ammonia reacts with NOx at reaction sites on first and second catalysts 130,116, reducing NOx in exhaust 122 to a predetermined level. Ammonia sensor 180 senses the level of ammonia entrained in exhaust gas leaving first catalyst 130 and sends a signal to SCM 154, which is being provided with various relevant engine operating parameters. SCM 154 is programmed with an algorithm describing exhaust ammonia levels as a function of engine operating data over the range of engine speeds and temperatures in which the engine can operate, as well as test-bed generated performance data showing ammonia storage three-dimensionally within first and second catalysts 130,116 as a function of at least exhaust temperature, exhaust flow rate, and NOx level. The SCM determines whether the level of ammonia being experienced in the exhaust by sensor 180 is within predetermined limits and also predicts, from the present urea dosing rate and the expected near-term engine conditions, whether the present ammonia level is a) insufficient, b) correct, or c) excessive to meet present and anticipated needs in order to meet desired levels of NOx and ammonia in tailpipe exhaust 122. The SCM then adjusts the liquid urea dosing rate accordingly and begins another calculation cycle.

While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims. 

1. A system for control of nitrogen oxide concentrations in a stream of exhaust gas from an internal combustion engine, comprising: a) a first catalytic element disposed in said stream of exhaust gas for catalytically reducing nitrogen oxides to elemental nitrogen; b) a second catalytic element disposed in said stream of exhaust gas downstream of said first catalytic element for further catalytic treatment of said exhaust gas; c) a system for injecting an ammoniacal chemical reductant into said stream of exhaust gas ahead of said first and second catalytic elements; and d) an ammonia sensor disposed in said stream of exhaust gas between said first and second catalytic elements for sensing ammonia concentrations in said stream of exhaust gas.
 2. A system in accordance with claim 1 wherein said further catalytic treatment is selected from the processes consisting of reducing nitrogen oxides, oxidizing ammonia, and combinations thereof.
 3. A system in accordance with claim 1 wherein said first and second catalytic elements are disposed in a single housing.
 4. A system in accordance with claim 1 wherein said first and second catalytic elements are provided in a single monolithic catalytic brick, and wherein said ammonia sensor is disposed at an intermediate longitudinal location of said brick, which location defines portions of said brick upstream of said ammonia sensor as said first catalytic element and portions of said brick downstream of said ammonia sensor as said second catalytic element.
 5. A system in accordance with claim 1 wherein said first catalytic element and second catalytic element are abuttingly disposed and have a well formed at the location of said abutting, and wherein said ammonia sensor is disposed in said well.
 6. A system in accordance with claim 1 wherein said first catalytic element and said second catalytic element have a gap therebetween, and wherein said ammonia sensor is disposed in said gap.
 7. A system in accordance with claim 1 wherein said ammoniacal chemical reductant is ammonia.
 8. A system in accordance with claim 7 wherein said ammonia is derived by decomposition of urea.
 9. A system in accordance with claim 8 wherein said urea is provided in the form of an aqueous solution.
 10. A system in accordance with claim 1 further comprising a programmable controller responsive to signals from said ammonia sensor for regulating rate of injecting of said ammoniacal chemical reductant into said exhaust gas stream.
 11. A system in accordance with claim 1 further comprising a second ammonia sensor disposed in said stream of exhaust gas after said second catalytic element.
 12. An internal combustion engine, comprising a system for control of nitrogen oxide concentrations in a stream of exhaust gas from said engine, wherein said system includes a first catalytic element disposed in said stream of exhaust gas for catalytically reducing nitrogen oxides to elemental nitrogen, a second catalytic element disposed in said stream of exhaust gas downstream of said first catalytic element for further catalytic treatment of said exhaust, a system for injecting an ammoniacal chemical reductant into said stream of exhaust gas ahead of said first and second catalytic elements, and an ammonia sensor disposed in said stream of exhaust gas between said first and second catalytic elements for sensing ammonia concentrations in said stream of exhaust gas.
 13. An internal combustion engine in accordance with claim 12 wherein said further catalytic treatment is selected from the processes consisting of reducing nitrogen oxides, oxidizing ammonia, and combinations thereof.
 14. An internal combustion engine in accordance with claim 12 wherein said engine is selected from the group consisting of spark-ignited and compression-ignited. 